[Action] Apply passive chain multiplier modifiers without chain targets

[Nnoggie]

Clawing Shadows (1241567) effect #3 reduces the chain multiplier of Scourge Strike, Clawing Shadows and Vampiric Strike by 10% ("Each subsequent chain deals 10% reduced damage"). Currently every chained hit deals full damage.

Cause

#10659 removed the per-action P_CHAIN_MULTIPLIER handling in apply_affecting_aura (chain_multiplier *= 1.0 + effect.percent()). The passive system now writes the modifier into the spell data, which works:

Clawing Shadows (1241567) eff#3 modifying Scourge Strike (55090) chain_multiplier by -10% (orig=1 prev=1[0/100%] now=0.9[0/90%])
Clawing Shadows (1241567) eff#4 modifying Scourge Strike (55090) chain_target by 1 (orig=0 prev=0[0/100%] now=1[1/100%])

The action only reads that value in action_t::parse_effect_data, inside if ( spelleffect_data.chain_target() > 1 ). Scourge Strike has 0 chain targets in spell data, 1 with the talent, and gets its extra targets from the Clawing Shadows buff through n_targets(). The action therefore keeps chain_multiplier = 1.0.

In current data, Clawing Shadows is the only passive chain multiplier modifier on a spell below that threshold. Accelerated Blade's Throw Glaive effects have 2-3 chain targets and already work.

Change

For effects below the threshold, parse_effect_data now takes the passive percent modifier from get_passive_value( effect, "chain_multiplier" ) on its own, as the pre-#10659 handling did. The base spell data chain multiplier stays unused for these spells. 13 damage effects have 1 chain target and a non-1 multiplier (Death Strike, Chaos Strike, Annihilation, Judgment, Hammer of Wrath, Pyroblast, Phoenix's Flames, Icicle, Black Icicle). Their modules handle multi-target behavior themselves, and without a passive modifier they are unchanged.

The value is assigned rather than multiplied because the passive system writes the modifier to every effect of the spell with a non-zero chain multiplier, including Scourge Strike's dummy effects.

Verification

• MID1_Death_Knight_Unholy.simc, desired_targets=5, debug=1, average_range=1: in one Scourge Strike that hits all 5 targets, raw_amount / tgt_da_mul per chain is 15562.7, 14006.4, 12605.8, 11345.2, 10210.7, a 0.900 ratio at every step. Before this change all chains were equal. Vampiric Strike on San'layn shows the same 0.9 step.
• Deterministic (deterministic=1, 200 iterations) DPS of every loadable MID1 and MID2 profile at 1 and 5 targets: 156 sims, all identical to a build of this branch's base with only a Death Knight module-side fix (chain_multiplier = data().effectN( 1 ).chain_multiplier() in scourge_strike_base_t). Outside Death Knight that build is the unmodified base, so no other spec changes; the MID2 profiles cover the 13 effects above. Unholy matches the module-side fix exactly.

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